1. Field of the Invention
The present invention generally relates to the processing of waste water, and more specifically to its defluoridation. The present invention more specifically addresses effluents from silicon processings.
2. Discussion of the Related Art
The electronic industry increasingly uses silicon-based semiconductor devices. During the manufacturing processes of these devices, etching of the silicon have, in particular, to be performed by means of components containing fluorine, such as, for example, hydrofluoric acid (HF) or ammonium fluorides (NH4F), which produce at the end of a process, effluents with a high concentration of fluoride ions. Such materials are particularly harmful for the environment and, accordingly, these effluents have to be defluoridated in a sewerage plant. Of course, other silicon processing is performed during manufacturing. The different effluents are gathered before being processed by the sewerage plant.
The effluents typically have concentrations in fluoride ions ranging from several tens to several hundreds of milligrams per liter, which concentrations are higher than the maximum non-polluting limit, set, for example, to 15 mg/l. To efficiently reduce, in an economically viable way, this high fluoride ion concentration in an industrial process, a physico-chemical processing based on milk of lime (calcium hydroxide Ca(OH)2 in suspense in water) is used, which is based on the spontaneous precipitation, in the form of calcium fluoride (CaF2), of the fluoride ions in the presence of calcium ions according to the following equilibrium:
2Fxe2x88x92+Ca++⇄CaF2.xe2x80x83xe2x80x83(equilibrium 1)
Dead lime has the advantages of introducing calcium ions in the solution, while ensuring, by the addition of two hydroxide ions (OHxe2x88x92) for one calcium ion (Ca++), according to the following equilibrium:
Ca(OH)2⇄2OHxe2x88x92+Ca++,xe2x80x83xe2x80x83(equilibrium 2)
a basicity of the medium which optimizes the precipitation.
Indeed, for fluoride ions, the precipitation is optimal, in the presence of dead lime, in a pH interval between 8 and 9.
The conventional processing essentially includes five steps.
FIG. 1 very schematically shows an example of known sewerage plant, applied to defluoridation.
In a first step, the waste water ww is mixed with a compound liberating dissolved calcium and with a compound likely to capture dissolved protons (H+). Preferably, dead lime s1 brought from a vessel 10, which answers both requirements, is used. This step will be called hereafter the basic neutralization, or basification, and enables precipitation of the fluoride ions.
In the example shown, this first step is divided into a pre-neutralization and a post-neutralization, performed until respective pH values on the order of approximately 6.5 and 9 are reached, which values are automatically controlled, for example, by a pH probe.
The initial pre-neutralization eliminates the excess fluoride ions which may be present in waste water ww before the pH increase of the post-neutralization which favors their appearance.
The pre-neutralization step is a step of mixture with lime s1 in a processing vessel 11 until a pH on the order of 6.5 to 7 is reached. The post-neutralization step is a step of reaction and pH adjustment to a value on the order of 8 to 9 in a processing vessel 12.
During the post-neutralization, all fluorine salts are assumed to dissociate and all fluoride ions are assumed to precipitate due to the excess of lime present.
The basification step is generally followed by a flocculence step which consists of introducing flocculents fa, coming from a vessel 16, to help a subsequent decantation. This flocculence step can be implemented in a settling tank 12xe2x80x2, or in a separated vessel.
A decantation step is then performed in a decanter 13.
The liquid phase lp floating on the surface (not precipitated) is extracted from decanter 13. According to the precipitation principles of equilibrium 1, all fluoride ions are assumed to be precipitated by the excess lime and to be held by the decanter in sludge, or solid phase sp. Sludge sp is removed from the decanter, at periodic intervals, while liquid phase lp is extracted continuously.
Although it has not been shown, a low quantity of the sludge is generally brought back to processing vessel 12 to xe2x80x9cseedxe2x80x9d it with CaF2 crystals, to favor the formation of the precipitate (CaF2) in the post-neutralization step.
The fluoride content of the effluents to be rejected, that is, of liquid phase lp, is then tested. These tests, generally represented by an intermediary vessel 14, enable eventual adjustment of the final pH of the rejected water which must be substantially neutral (pH≈7), to control the ion concentrations of this liquid phase, and possibly to decide a new processing.
Finally, the effluents are removed, for example, in an urban sewage system 15.
Generally, additional recycling separation steps (not shown) are performed, preferably in combination with the tests.
The different transfers and their flow rates between the processing and/or storage vessels are controlled conventionally by pumping and/or flow methods (not shown) controlled by well known techniques, for example, electrovalves provided with usual security and manual and/or automatic control systems.
The different vessels (storage and/or processing) are also equipped with conventional mixing means to homogenize the different suspensions or solutions in temperature, concentrations, and pH.
The limits of such a dead lime method essentially appear from the combination of the two equilibriums 1 and 2.
In the presence of an excess of calcium ions, due to the excess of lime enabling, besides, to obtain a basic pH at the end of the neutralization (here, pH=9), the precipitation is only limited by the solubility limit of the calcium fluoride in the solution. Accordingly, this method allows, theoretically, reaching a residual concentration of fluoride ions of 8 mg/l.
In practice, higher final fluoride ion concentrations, on the order of 17 to 24 mg/l, can be observed.
An object of the present invention is to provide a novel method of defluoridation which results in a residual concentration of fluoride ions lower than 15 mg/l.
Another object of the present invention is to provide a fast, non-polluting method which allows use of already existing structures.
A first solution would be to increase the time of contact between the effluents and the lime (pre-neutralization and post-neutralization), considering that, even in the presence of a lime excess, equilibrium 1 moves very slowly towards the formation of the calcium fluoride precipitate (CaF2). A prolongation of this time of exposition to lime has been found to be ineffective.
The present invention originates from a novel approach of the phenomena leading to a partial defluoridation.
The present inventors consider that, at its arrival at the level of the sewerage plant, the waste water contains different fluoric salts.
A portion of these fluoric salts, which will here be called xe2x80x9csimplexe2x80x9d salts, that is, having simple ionic groups such as, for example, ammonium fluoride (NH4F), sodium fluoride (NaF), or even isolated fluoride ions due to an excess of hydrofluoric acid (HF), is dissociated in the conditions (pH) of the processing, and thus leads to a precipitation of fluoride ions.
The present inventors consider that other salts, which will here be designated as xe2x80x9ccomplexxe2x80x9d salts, coming from more elaborate combinations of the effluents which can even involve phenomena of complexation in the presence of metal ions, are not dissociated in the present neutralization conditions.
Thus, a characteristic of the present invention is to provide the dissociation of xe2x80x9ccomplexxe2x80x9d salts.
More specifically, the present invention provides a method of defluoridation of waste water, including a step of acid neutralization between a basic neutralization step and a decantation step.
According to an embodiment of the present invention, the acid neutralization step is performed by addition of an acid other than hydrofluoric acid (HF).
According to an embodiment of the present invention, the acid neutralization step is performed until a substantially neutral or very slightly acid pH is reached.
According to an embodiment of the present invention, the waste water comes from silicon processing.
According to an embodiment of the present invention, the acid neutralization step is performed by addition of sulfuric acid (H2SO4).
According to an embodiment of the present invention, the method implements the following successive steps:
basic pre-neutralization until a pH value of 6.5 is reached, by mixing the waste water with dead lime (Ca(OH)2);
basic post-neutralization until a pH value of 8.2 is reached, by mixing the pre-neutralized solution with dead lime;
acid neutralization until a pH value included between 5.5 and 7 is reached;
decantation coupled with operations of continuous extraction of the liquid phase; and
removal of the defluorided effluents.
The present invention also provides a method of purification of waste water, including a step of acid neutralization.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.